Permanent magnet machines operate in a well known fashion, utilizing a non-stationary member called the rotor securing a plurality of magnets and a stationary member entitled the stator embedded with a plurality of wire coils with which the rotor elements electro-magnetically interact. Torque is communicated to or from the rotor relative to the stator mechanically by means of a shaft that is co-axial to both the rotor and the stator.
Where the permanent magnet apparatus serves as an electrical alternator, torque is transferred to the rotor elements from an external power source for purposes of inducing electrical currents in the wire coils of the stator elements so that mechanical work is converted into electrical energy. Where, on the other hand, the apparatus is an electric motor, torque is transferred from the rotor elements to the shaft for the performance of external work by being magnetically driven when electrical current passes through the stator wire coils.
The focus of recent technological developments in alternator design has been upon higher shaft speeds. This achieves more efficient electrical output at greatly decreasing diameters in the size of the alternator. Such alternators can operate at speeds as high as 24,000 revolutions per minute (rpm). In particular, this focus has led to advances in the very materials used in the alternators, materials such as magnetic steel alloys and permanent magnets. Although the expense associated with these improved materials is quite high, their use in high speed alternators is still cost-effective since they are able to achieve improved performing alternators despite utilizing less material.
Another type of alternator, one carrying torque from a relatively less intense power source, operates instead at shaft speeds ranging from 300 rpm to as low as 20 rpm. With these low-shaft-speed alternators, the increased cost in materials becomes highly significant since these machines, due to their greater overall diameter, require much more material for an increased number of stator and rotor elements than their high speed counterparts. The larger diameter of these devices is unavoidably necessary if the magnets are to achieve the level of speed or tangential velocity needed for the rotor to effectively induce the desired electrical output.
Conventional low-speed alternators typically utilize rotors constructed from a solid cylindrical band of magnetic steel. Unlike high-speed alternators whose rotors are assembled from laminated circular plates stamped from thin sheets the width of the rotor, the comparatively larger diameter of the low-speed devices makes a similar process prohibitively expensive since the stamping required to form such plates needs extremely large and expensive tooling, leaving a great amount of wasted material as well.
In addition, electrical alternators often represent a significant portion of the overall cost of any power-generating system. Alternator expense can have a profound effect on the ultimate success or failure of a particular design for such a system. Total manufacturing, labor and material costs therefore impact not only upon the ultimate price of the alternator but also upon the viability of the power-generating apparatus in which it will be used.
Wind-generated power devices are one example of these power-generating systems. Wind-generated power is a needed and highly desirable alternative to power created by utilities using coal, natural gas or other non-renewable sources. Over the past two years, the United States has increased its wind-power capacity faster than any other country with wind farms now operating in at least 36 states. Wind farms generate electricity by using wind to turn giant blades that, in turn, rotate wind turbines. A recent study has predicted that wind farms may be generating 7% of the nation's electricity by the year 2023.
A wind turbine mechanically connects the shaft on which the blades turn to an alternator. Wind turbines can range in size of their electrical output from several megawatts to less than one kilowatt. The latter turbines are often used for homes in locations where a connection to the utility grid is not available or not desired.
The shaft to a wind turbine rotates, however, at a speed ranging from just 30 to 60 rpm. A gear box is therefore required in order to increase the rotational speed to at least 1000 to 1800 rpm since these are the speeds needed for the turbine to utilize high-speed alternators. Since they can operate at the low shaft speeds of the wind turbine, low-speed alternators, on the other hand, are considered “direct-drive” alternators. As a consequence, these devices at least eliminate the need for a costly and heavy gear box, reducing not only the amount of maintenance required by a turbine but also removing the complexity and energy loss associated with this particular transmission process.
While the electrical frequencies produced by direct-drive alternators may be quite low, the efficiency losses associated with higher frequency/high-speed alternators are, however, not as pronounced. A low-speed alternator that is low in cost and easily assembled would therefore be highly desirable for use with wind turbines as well as other similarly cost-sensitive power-generating systems.